Isotopic tracers in Oceanography

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1 Isotopic tracers in Oceanography

2 Isotopic tracers in oceanography Pathways of different trace metals from their sources to their sinks M. Frank: Reviews of Geophysics, vol. 40 DOI: /2000RG

Pb isotopes in the ocean Chow & Patterson 1959, 1962: First major Pb isotope studies on manganese nodules and pelagic sediments 206 Pb/ 207 Pb Result: General distinction between the Pb isotope

3 Pb isotopes in the ocean Chow & Patterson 1959, 1962: First major Pb isotope studies on manganese nodules and pelagic sediments 206 Pb/ 207 Pb Result: General distinction between the Pb isotope signatures of Atlantic and Pacific samples. Within each ocean, manganese nodules and pelagic sediments give relatively consistent results Conclusions: Pb has a relatively short residence time in seawater. Distinct Pacific and Atlantic Ocean signatures reflect average Pb isotope composition of the continents surrounding each ocean from Goldstein & Hemming: Treatise on Geochemistry, Vol.6

4 Pb isotopes in the ocean Correspondence between lead ore compositions ( ) and gasolines ( ) from different countries (Chow 1970) from Alan P. Dickin:

5 Nd isotopes in the ocean Nd has very short residence time in seawater (different from Sr) Tracer for water mass circulation and for changes in ocean circulation pattern 143 Nd/ 144 Nd ratios allow identification of different source reservoirs from Goldstein & Hemming: Treatise on Geochemistry, Vol.6

6 Nd isotopes in seawater from Goldstein & Hemming: Treatise on Geochemistry, Vol.6

7 Nd isotopes in seawater from Goldstein & Hemming: Treatise on Geochemistry, Vol.6

8 Nd isotopes in seawater All rivers plotted drain a variety of continental crusts. Relatively low Nd ratio, relatively high Sr ratio Atlantic drains continental crust silicates (low Nd ratio) Pacific drains rocks that were mantle derived volcanics (high Nd ratio)

9 Nd isotopes in seawater S N AAIW NADW from Goldstein & Hemming: Treatise on Geochemistry, Vol.6

10 Nd isotopes in seawater from Alan P. Dickin:

11 Nd isotopes in seawater Comparison of seawater isotope compositions with sea floor Fe-Mn nodules and possible source reservoirs. seawater manganese nodules crustal sources Conclusion: Nd in Atlantic seawater is primarily continental in origin. Consistent with greater discharge of river water into the Atlantic. ~50% of Nd in Pacific seawater appears to be derived from basaltic crustal sources, either from erosion of mafic rocks or from some form of exchange with ocean floor basalts. from Alan P. Dickin:

12 Nd isotopes in seawater increasing contribution from continental weathering From ferromanganese oxyhydroxide coatings on the foram tests. Show regional changes in the S. Atlantic Basin. mantle-derived (volcanic input) From foram tests CaCO 3 Global change

13 Nd isotopes in seawater Rise of volcanic (mantle source) inputs to the Pacific Some differences due to bottom water inputs deep circulation. it is the short residence time of Nd in the ocean relative to ocean mixing rates that allows to get basin specific information

14 Nd isotopes in seawater Paleo-seawater Nd curves (derived from conodonts, fish debris, forams or BIFs) Phanerozoic Archean & Proterozoic from Alan P. Dickin:

15 Panama isthmus started to close ca. ~5 Ma ago. Nd isotopes in seawater

16 Nd isotopes in seawater Nd-(and 18 O)- isotopes in foraminifera, Indian ocean: monsoonal activity Nd-isotopes Pacific & Atlantic ocean: closure of Panama gateway at 10-3 Ma. Nd isotopes S- Atlantic: proxy for NADW.

17 Nd-Hf isotope ratios in rocks and Fe-Mn deposits Hf: shows relative homogeneous signature in seawater. Mixing between crustal (negative) and mantle (basaltic crust, pos. Hf values) sources. Crustal source reflects composition of non-zirconbearing sediments. Mantle-like end member with high εhf (~ +16). Deduced range of seawater Hf is much smaller than range between end-members Hf must have a long residence time in seawater (longer than Nd, shorter than Sr), to allow such high degree of homogenisation. from Goldstein & Hemming (2004): Treatise on Geochemistry, Vol.6

18 Sr in the oceans 87 Sr/ 86 Sr of modern (and ancient) seawater is homogeneous on a global scale. Residence time (~4 Ma, Hodell et al. 1990) is long compared to mixing time (1-2 ka) Sr isotope composition of the oceans is determined by the relative contributions of Sr from river waters and hydrothermal vents Burke et al DePaolo 1987 Based on brachiopods, belemnites, conodonts, foraminifera, micritic matrix

19 U-series disequilibrium methods Potts 1987

20 U-series disequilibrium methods 238 U 234 U 230 Th 206 Pb In case of secular equilibrium: λ 1 N 1 = λ 2 N 2 A 1 = A 2 or A i = λ i N i e.g. ( 230 Th/ 238 U) = 1 Stosch:

21 U-series disequilibrium methods In case of secular equilibrium: λ 1 N 1 = λ 2 N 2 A 1 = A 2 or A i = λ i N i e.g. ( 230 Th/ 238 U) = 1

22 U-series disequilibrium methods 238 U 234 U 230 Th 206 Pb Isotopes in the decay series can become fractionated relative to one another, due to variations in their chemistry (seawater) or the structural site (crystal) secular disequilibrium. Such a situation can be utilised in two different ways as a dating tool, the daughter-excess and daughterdeficiency dating methods. The concentration in excess of the equilibrium is called unsupported or an excess of a nuclide

23 U-series disequilibrium methods 238 U 234 U 230 Th 206 Pb (excluding some shorties ) T 1/2 = 75 ka A Equilibrium: λ 1 N 1 = λ 2 N 2 A 1 = A 2 ( 230 Th/ 238 U) = 1 A Allègre & Condomines (1976) EPSL 28, (1982) Nature 299

24 U-Serien-Ungleichgewichtsmethoden 238 U 234 U 230 Th 206 Pb 222 Rn 210 Pb 222 Rn 238 U 234 U 230 Th 226 Ra Gelöst in Meerwasser U: ~ 3 ppb Th: < ppb In oxidized waters, U is soluble as U(VI), Uranyl ions = UO 2 ++, strong complexes with carbonates Th is present as Th(IV); highly insoluble; sorbs to surfaces Residenzzeit: U: Ma Th: ~300 a 226 Ra Aufgrund der schnellen Sedimentation von Th aus dem Meerwasser weisen junge pelagische Gesteine einen 230 Th-Überschuß auf! 230 Th ( 231 Pa, 210 Pb, etc.)

25 230 Th/ 232 Th (ionium) method 238 U 234 U 230 Th 206 Pb 1. Preferential removal of Th from seawater 2. Freshly deposited sediment contains unsupported 230 Th 3. Decay of excess 230 Th in the sediment (see figures) 4. Allows to determine sedimentation rates R = R o e -λt T = 1/λ ln(r o /R) sedimentation rate: a = h/t (h = depth) R = R o e -λh/a lnr = lnr o λh/a 1 Normal pattern 2 change in sedimentation rate 3 Mixing of sediments (e.g. bioturbation) at top 4 dominance of U-supported 230 Th Stosch:

238 U- 234 U disequilibrium method Stosch: http://www.dmg-home.

pdf The activity of 234 U in seawater not in secular equilibrium with that of 238 U

26 238 U- 234 U disequilibrium method Stosch: The activity of 234 U in seawater not in secular equilibrium with that of 238 U 234 U is preferentially leached out of radiation damaged crystal lattice sites during weathering The 234 U A / 238 U A ratio of sea water is about 1.15

27 238 U- 234 U disequilibrium method Activity disequilibrium between 234 U and 238 U isotopes in seawater G. Faure: Principles of Isotope Geology U/ 238 U activity ratio of seawater is ~1.15 (contains excess 234 U) 2. When CaCO 3 is produced by coral or foraminifera it incorporates some U from seawater but almost no Th 3. When e.g. a coral is separated from seawater, excess 234 U decays into 230 Th The limit of the dating range is reached when secular equilibrium between 234 U and 238 U is established

28 230 Th- 238 U and 230 Th- 234 U methods 238 U 234 U 230 Th 206 Pb Th/U isotope ratio can be used to date carbonates provided we know: (1) the initial 234 U/ 238 U ratio (2) that the initial 230 Th/ 238 U ratio was close to zero and that the sample has remained closed with respect to U, Th and their intermediate isotopes Exponential decay towards equilibrium Ingrowth of 230 Th provides an age Must account for initial Th (by measuring 232 Th) and initial seawater 234 U/ 238 U

GEOL 562 Notes: U-Series and Th-series nuclides. Guide Questions: Reading: White, Lecture 10

GEOL 562 Notes: U-Series and Th-series nuclides Reading: White, Lecture 10 Motivation: Up to now we have dealt with long half-life nuclides (all left over from the birth of the solar system). What are